I found these two today. They were on an overgrown plot - which are very interesting to hang round, but not very welcome on an allotment site!
This is an orange tip butterfly caterpillar. It's long and thin and velvet looking. It generally eats weeds, including red campion that we have on our plot, amongst other wildflowers.
Up close you can see that its legs are darker green - they looked like part of the stem it was standing on when I took the photo.
This is a mullein moth caterpillar.This is pretty small, at an early part of its larval stage, they grow quite large before pupating. As before when I saw one, it's not actually on a mullein plant, but it shouldn't cause a problem to allotment plants.